What do you think happened here?

Hi, at the end of September I bought several chickens (6-8 months old) at a poultry auction (I know, probably not the best move, in retrospect). Last Wednesday, one of them (in quarantine from our other barn of chickens) came down with something. I isolated her. Her neck was twisted, and it made it hard for her to eat and drink. She was depressed and inactive, but could still stand. Gradually she went downhill and avoided the small heat lamp I put for her. She had dropped at least half her weight. She was hanging on, but as it seemed that the end was inevitable, finally on Saturday, I asked my husband to put her out of her misery. When he picked her up, we noticed an opaque, colorless discharge issuing from her nostrils. The twisted neck, depression, and nostril discharge seem to have been the only symptoms. Does anyone have an idea of what this might have been?

We are disinfecting the barn today. Everyone else seems just fine. Thanks in advance for any guesstions on what hit our poor girl.